Biaxially oriented polyester film and production process therefor

ABSTRACT

A biaxially oriented polyester film for a high-density magnetic recording medium, particularly a high-density magnetic recoding medium of linear recording system, which has high strength in longitudinal and transverse directions, excellent dimensional stability in a crosswise direction and is flat and excellent in output characteristics and a process for producing the same. The biaxially oriented film is made from polyethylene-2,6-naphthalate and has a Young&#39;s modulus in a longitudinal direction of 8 GPa or more, a Young&#39;s modulus in a transverse direction of 6 GPa or more, a temperature expansion coefficient in the transverse direction (αt) of −5×10 −6 /° C. to +12×10 −6 /° C., a humidity expansion coefficient in the transverse direction (αh) of 5×10 −6 /% RH to 12×10 −6 /% RH and a thermal shrinkage factor in the transverse direction at 105° C. of −0.5 to +1.5%.

This is a divisional of application Ser. No. 10/203,346 filed Aug. 9,2002, now U.S. Pat. No. 6,890,471.

FIELD OF THE INVENTION

The present invention relates to a biaxially oriented polyester film anda production process therefor. More specifically, it relates to abiaxially oriented polyester film which has excellent dimensionalstability while retaining high Young's moduli and is therefore useful asa base film for high-density magnetic recording media, particularly LTOand S-DLT magnetic tapes of linear recording system, and a productionprocess therefor.

DESCRIPTION OF THE PRIOR ART

A polyester film is used in a wide variety of fields such as magneticrecording media and electrical insulation as it has excellent thermaland mechanical properties. As the capacity and density of a magneticrecording medium, particularly a data storage tape have been increasingin recent years, requirements for a base film for use in the medium havebeen becoming higher and higher.

In order to ensure a large capacity for a tape, it is conceivable toreduce the thickness, extend the length, flatten the magnetic side orincrease the linear recording density or the number of tracks of thetape. A base film having higher flatness, higher strength and excellentdimensional stability in a crosswise direction is desired.

Heretofore, a polyethylene terephthalate film has been widely used as abase film for magnetic tapes but a polyethylene-2,6-naphthalenedicarboxylate film having high strength and high dimensional stabilityhas recently been used very often. However, when the strength in thelongitudinal direction of the film is to be increased, the strength inthe transverse direction lowers, resulting in deteriorated dimensionalstability in the crosswise direction. Also, when the strength in thetransverse direction is to be increased to improve dimensional stabilityin the transverse direction, the strength in the longitudinal directionlowers. Thus, a biaxially oriented polyethylene-2,6-naphthalenedicarboxylate film which has excellent dimensional stability whileretaining high Young's moduli in both longitudinal and transversedirections is yet to be provided.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a biaxially orientedpolyester film which solves the above problem, has high strength in bothlongitudinal and transverse directions and excellent dimensionalstability in the crosswise direction and comprisesethylene-2,6-naphthalene dicarboxylate as the main recurring unit.

It is another object of the present invention to provide a flatbiaxially oriented polyester film which is useful as a base film forhigh-density magnetic recording media having excellent outputcharacteristics, particularly high-density magnetic recording media oflinear recording system.

It is still another object of the present invention to provide anindustrially advantageous process for producing a biaxially orientedpolyester film having the above excellent properties of the presentinvention.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention are attained by a process forproducing a biaxially oriented polyester film, comprising the steps of:

-   (1) stretching an unstretched film of a polyester which comprises    ethylene-2,6-naphthalene dicarboxylate in an amount of at least 95    mol % of the total of all the recurring units to 4.5 to 7.0 times in    a machine direction at a temperature of 100 to 190° C. to form a    uniaxially oriented film; and-   (2) stretching this uniaxially oriented film to 4.0 to 7.0 times in    a transverse direction at a temperature of 110 to 170° C. while    raising the temperature in the traveling direction of the film and    then stretching the film to 1.05 to 1.5 times at a lower draw rate    than the first draw rate at a temperature from the final temperature    of the first transverse orientation to 240° C. while raising the    temperature in the traveling direction of the film to form a    biaxially oriented film having (i) a Young's modulus in the    longitudinal direction of 8 GPa or more, (ii) a Young's modulus in    the transverse direction of 6 GPa or more, (iii) a temperature    expansion coefficient in the transverse direction (αt) of    −5×10⁻⁶/° C. to +12×10⁻⁶/° C., (iv) a humidity expansion coefficient    in the transverse direction (αh) of +5×10⁻⁶/% RH to +12×10⁻⁶/% RH,    and (v) a thermal shrinkage factor at 105° C. in the transverse    direction of −0.5 to +1.5%.

According to the present invention, secondly, the above objects andadvantages of the present invention are attained by a biaxially orientedpolyester film which has (i) a Young's modulus in a longitudinaldirection of 8 to 12 GPa, (ii) a Young's modulus in a transversedirection of 6.5 to 9 GPa, (iii) a temperature expansion coefficient inthe transverse direction (αt) of −5×10⁻⁶/° C. to +12×10⁻⁶/° C., (iv) ahumidity expansion coefficient in the transverse direction (αh) of+6×10⁻⁶/% RH to +12×10⁻⁶/% RH and (v) a thermal shrinkage factor at 105°C. in the transverse direction of 0 to +1.5% and which comprises (vi)ethylene-2,6-naphthalene dicarboxylate in an amount of at least 95 mol %of the total of all the recurring units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a device for measuring a dimensionalchange in a crosswise direction under load in a longitudinal direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will be described in detail hereinbelow. Adescription is first given of the production process. As describedabove, the process of the present invention comprises the steps ofstretching a film in a machine (longitudinal) direction and thenstretching the film in a transverse direction in two stages, the drawrate of the second-stage transverse orientation being lower than thedraw rate of the first-stage transverse orientation.

The unstretched film to be uniaxially stretched in the step (1) is madefrom a polyester which comprises ethylene-2,6-naphtahlene dicarboxylatein an amount of at least 95 mol % of the total of all the recurringunits. The polyester is particularly preferably a homopolymer ofethylene-2,6-naphthalene dicarboxylate.

The polyester can be produced by a method known per se. For example, itcan be produced by carrying out an ester exchange reaction between alower alkyl ester of 2,6-naphthalenedicarboxylic acid and ethyleneglycol and polycondensing the reaction product. The ester exchangereaction catalyst used for the ester exchange reaction is preferably amanganese compound, and the manganese compound is preferably an oxide,chloride, carbonate or carboxylate, particularly preferably manganeseacetate. When the ester exchange reaction is substantially completed, aphosphorus compound is preferably added to deactivate the ester exchangecatalyst. The phosphorus compound is preferably trimethyl phosphate,triethyl phosphate, tri-n-butyl phosphate or orthophosphoric acid,particularly preferably trimethyl phosphate. The polycondensationcatalyst is preferably an antimony compound, particularly preferablyantimony trioxide.

The intrinsic viscosity of the thus obtained polyester is preferably0.40 (dl/g) or more, more preferably 0.40 to 0.90. When the intrinsicviscosity is less than 0.4, the film often breaks in the stretchingstep. When the intrinsic viscosity is more than 0.9, the polymerizationproductivity of the polyester tends to lower disadvantageously. Aftermelt polymerization, the polyester may be chipped and solid-phasepolymerized under vacuum heating or in a stream of an inert gas such asnitrogen.

Various additives may be added to the above polyester used in thepresent invention in limits not prejudicial to the object of the presentinvention. Particularly addition of inert fine particles is desired toadjust the surface roughness of the obtained biaxially orientedpolyester film to a suitable range. Addition of the inert fine particleswill be described hereinafter.

In the step (1), the unstretched film of the above polyester isstretched to 4.5 to 7.0 times in the machine direction at a temperatureof 100 to 190° C. to form a uniaxially oriented film. When the drawratio stretching in machine direction is lower than 4.5 times, theYoung's modulus in the longitudinal direction of the finally obtainedbiaxially oriented film tends to fall below 8 GPa and when the drawratio stretching in machine direction is higher than 7.0 times, the filmis easily broken by stretching in the transverse direction in thesubsequent step (2), thereby making it difficult to adjust the Young'smodulus in the transverse direction of the finally obtained biaxiallyoriented film to 6 GPa or more.

The temperature for stretching in machine direction of the step (1) ispreferably 120 to 170° C. and the draw ratio is preferably 5.0 to 6.5times.

The uniaxially oriented film obtained in the step (1) has a refractiveindex in the longitudinal direction (NMD) of 1.77 or more, a refractiveindex in the transverse direction (NTD) of 1.55 to 1.62, preferably 1.57to 1.60, and a refractive index in the thickness direction (NTD) ofpreferably 1.50 to 1.56, particularly preferably 1.52 to 1.56. When therefractive indices of the uniaxially oriented film are outside the aboveranges, the film is often broken by stretching in the transversedirection in the subsequent step, or a biaxially oriented film havingthe targeted Young's moduli is hardly obtained.

In the step (2), the uniaxially oriented film is first stretched to 3.0to 6.0 times in the transverse direction at 110 to 170° C. while thetemperature is raised in the traveling direction of the film, namely,the temperature of the film is raised in the traveling direction of thefilm (first transverse stretching). When the above draw ratio in thetransverse direction is lower than 3 times, a biaxially oriented filmhaving the targeted Young's modulus in the transverse direction ishardly obtained and when the draw ratio is higher than 6 times, the filmis often broken, resulting in greatly deteriorated film formingproperties.

The temperature of the first transverse orientation in the step (2) ispreferably 130 to 160° C. and the draw ratio is preferably 4.0 to 5.0times. As for the elevation of the temperature in the travelingdirection of the film for the first transverse stretching, the gradientof a temperature rise is preferably in the range of 15 to 55° C., morepreferably 20 to 50° C. during the first transverse orientation.

In the step (2), the first transverse stretching is followed by furtherstretching the film to 1.05 to 1.5 times in the transverse direction ata lower draw rate than the draw rate of the first transverse stretchingat a temperature from the same temperature as the final temperature ofthe first transverse stretching to 240° C. while raising the temperaturein the traveling direction of the film (second transverse stretching).When the draw ratio of the second transverse stretching is lower than1.05 times, the draw ratio of the previous transverse stretching must bemade excessively high to obtain the targeted Young's modulus in thetransverse direction of the finally obtained biaxially oriented film. Inthis case, the film is readily broken by the previous transversestretching. When the draw ratio of the second transverse stretching ishigher than 1.5 times, the thermal shrinkage factor in the transversedirection of the obtained biaxially oriented film becomes too largedisadvantageously.

The temperature of the second transverse stretching is preferably fromthe same temperature as the final temperature of the first transversestretching to 220° C. and the draw ratio is preferably 1.05 to 1.2times.

As for the elevation of the temperature in the traveling direction ofthe film for the second transverse stretching, the gradient of atemperature rise is preferably in the range of 20 to 90° C., morepreferably 25 to 85° C. during the second transverse orientation.

The draw rate of the second transverse stretching is lower than the drawrate of the first transverse stretching which is 10 to 300%/sec, forexample, 0.1 to 30%/sec. The ratio of the draw rate of the firsttransverse stretching to the draw rate of the second transversestretching is preferably 0.005 to 0.5, more preferably 0.01 to 0.3, muchmore preferably 0.01 to 0.1.

In the process of the present invention, after the second transversestretching, it is preferred to further carry out the step of shrinkingor stretching the film to 0.9 to 1.05 times in the transverse directionat the same temperature as the final temperature of the secondtransverse orientation or at a temperature of 170 to 230° C. whilereducing the temperature from the final temperature in the travelingdirection of the film. When the temperature of this step, that is, heatsetting or crystallization step is lower than 170° C., the thermalshrinkage factor in the transverse direction of the film at 105° C.becomes too large and may exceed 1.5%. When the temperature is higherthan 230° C., the temperature and humidity expansion coefficients becomelarge and the dimensional stability in the crosswise direction isdeteriorated by temperature and humidity variations. As for adimensional change in the crosswise direction caused by final heatsetting, when the toe-out (elongation) is more than 10%, the thermalshrinkage factor becomes large, thereby deteriorating dimensionalstability in the crosswise direction and when the toe-in (shrinkage) ismore than 5%, the Young's modulus in the transverse direction of thefilm is suddenly lowered by this heat setting, thereby making itdifficult to obtain the required Young's modulus in the transversedirection. The total area draw ratio is preferably 20 to 50 times, morepreferably 25 to 45 times, particularly preferably 30 to 40 times.

As described above, by the process of the present invention, a biaxiallystretched (oriented) film having (i) a Young's modulus in thelongitudinal direction of 8 GPa or more, (ii) a Young's modulus in thetransverse direction of 6 GPa or more, (iii) a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient in the transversedirection (αh) of +5×10⁻⁶/% RH to +12×10⁻⁶/% RH, and (v) a thermalshrinkage factor at 105° C. in the transverse direction of −0.5 to +1.5%is formed.

The above biaxially oriented film is characterized in that its Young'smodulus in the longitudinal direction, Young's modulus in the transversedirection, temperature expansion coefficient in the transverse direction(αt), humidity expansion coefficient in the transverse direction (αh)and thermal shrinkage factor at 105° C. in the transverse direction arewithin the respective specific ranges.

This biaxially oriented film has a Young's modulus in the longitudinaldirection of the film of 8 GPa or more and a Young's modulus in thetransverse direction of the film of 6 GPa or more. When the Young'smodulus in the longitudinal direction is lower than 8 GPa and strongstress is applied to a magnetic tape, the tape elongates in thelongitudinal direction and deforms disadvantageously. When the film isused in a magnetic recording medium of linear recording system having ahigh track density, the medium shrinks in the crosswise direction by itselongation in the lengthwise direction, thereby causing trackdislocation. The Young's modulus in the longitudinal direction ispreferably 8.5 GPa or more, more preferably 9 GPa or more.

When the Young's modulus in the transverse direction of the film islower than 6 GPa, the temperature and humidity expansion coefficients inthe transverse direction become large, whereby when it is used in amagnetic recording medium of linear recording system having a high trackdensity, the film shrinks or elongates in the crosswise direction bytemperature and humidity variations, thereby causing track dislocation,or when a thin tape (base thickness of 3 to 7 μm) is caused to runrepeatedly, the end portion of the tape is damaged and deforms into aseaweed-like shape or is bent by contacting to a guide for restrictingthe transverse direction of the tape in an extreme case, thereby greatlyimpairing the characteristic properties of the tape. The Young's modulusin the transverse direction of the film is preferably 6.5 GPa or more,more preferably 7 GPa or more.

Although the Young's moduli in both longitudinal and transversedirections are desirably high, when the film is used in a magneticrecording medium of linear recording system, the Young's modulus in thelongitudinal direction is preferably higher than the Young's modulus inthe transverse direction. This is because it is more important toprevent the tape from being deformed or broken by a load as a base filmfor a high-density magnetic recording medium is thin.

The above biaxially oriented film has a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., a humidity expansion coefficient in the transversedirection (αh) of 5×10⁻⁶/% RH to 12×10⁻⁶/% RH and a thermal shrinkagefactor at 105° C. in the transverse direction of −0.5 to +1.5%. When thetemperature expansion coefficient or the humidity expansion coefficientin the transverse direction is larger than the above range and the filmis used in a magnetic recording medium of linear recording system havinga high track density, the dimensional change in a crosswise directioncaused by temperature and humidity variations becomes large, therebycausing track dislocation and making it impossible to read data. Whenthe temperature expansion coefficient or the humidity expansioncoefficient in the transverse direction is smaller than the above range,since the Young's modulus in the transverse direction becomes high, itis difficult to retain a high Young's modulus in the longitudinaldirection. Therefore, when strong stress is applied to the magnetictape, the tape elongates and deforms disadvantageously. The thermalshrinkage factor at 105° C. in the transverse direction is preferably−0.5 to +1.0%, particularly preferably −0.5 to +0.7%. When the thermalshrinkage factor at 105° C. in the transverse direction is outside theabove range, in the step of forming a magnetic tape, the elasticity ofthe film becomes large, whereby the film may be wrinkled, coating maybecome nonuniform, or the film may not be calendered well in thecalendering step. When a magnetic tape is formed from the film, the tapeshrinks or elongates in the crosswise direction by a temperature rise inthe drive, thereby causing track dislocation and making it impossible toread data.

The above biaxially oriented film has a refractive index in thethickness direction (Nz) of preferably less than 1.490, more preferablyless than 1.487, much more preferably less than 1.485, particularlypreferably less than 1.483. When Nz is more than 1.490, surfaceorientation becomes low and it is difficult to achieve high Young'smoduli in both longitudinal and transverse directions.

The magnetic layer forming side of the above biaxially oriented film ispreferably flat to obtain excellent electromagnetic conversioncharacteristics. The surface roughness (WRa) of at least one side of thefilm is preferably 0.5 to 10 nm, more preferably 0.8 to 7 nm,particularly preferably 1 to 5 nm. When this surface roughness WRa ishigher than 10 nm, it is difficult to maintain electromagneticconversion characteristics required for a magnetic tape. When thesurface roughness WRa is lower than 0.5 nm, the friction coefficientbecomes too large, thereby making it extremely difficult to cause thefilm to run and roll the film. The side opposite to the magnetic layerforming side, that is, non-magnetic layer side of the above biaxiallyoriented film has a surface roughness of 1 to 20 nm, more preferably 2to 15 nm, particularly preferably 2 to 12 nm to obtain excellent runningproperties. When the surface roughness of the non-magnetic layer side islower than 1 nm, the winding properties and transfer properties of thefilm during the production and processing of the film are poor, therebymaking it difficult to use it. When the surface roughness of thenon-magnetic layer side is higher than 20 nm, the flatness of themagnetic layer side may be impaired, thereby deterioratingelectromagnetic conversion characteristics.

In order to obtain surfaces which differ from each other in surfaceroughness, for instance, two layers which differ from each other in theaverage particle diameter and amount of inert fine particles to be addedto form fine irregularities on the surface of the film may be laminatedtogether, or a different coating layer may be formed on one side or bothsides of the film. As a matter of course, if the surface roughnesses ofthe magnetic layer side and the non-magnetic layer side fall within theabove respective ranges, the surface roughness of the magnetic layerside may be made equal to the surface roughness of the non-magneticlayer side. In this case, a single-layer film can be easily produced.

A biaxially oriented film consisting of two layers is obtained by usingan unstretched laminated film consisting of two layers as theunstretched film to be stretched in the longitudinal direction in thestep (1) of the above production process of the present invention.

The inert particles to be added to the film layer on which the magneticlayer is to be formed have an average particle diameter of preferably0.05 to 0.7 μm, more preferably 0.1 to 0.3 μm, particularly preferably0.1 to 0.2 μm. The amount of the inert particles is preferably 0.001 to1 wt %, more preferably 0.005 to 0.5 wt %, particularly preferably 0.01to 0.2 wt %. When the average particle diameter of the inert particlesis smaller than 0.05 μm or the amount thereof is smaller than 0.001 wt%, winding properties or transfer properties in the processing stepdeteriorate. When the average particle diameter is larger than 0.5 μm orthe amount is larger than 1 wt %, electromagnetic conversioncharacteristics worsen.

Examples of the inert particles to be added to the film layer on themagnetic layer side include (1) heat resistant polymer particles(particles of at least one of crosslinked silicone resin, crosslinkedpolystyrene, crosslinked acrylic resin, melamine-formaldehyde resin,aromatic polyamide resin, polyimide resin, polyamide-imide resin,crosslinked polyesters, etc.), and fine particles of inorganic compoundssuch as (2) metal oxides (aluminum oxide, titanium dioxide, silicondioxide (silica), magnesium oxide, zinc oxide, zirconium oxide, etc.),(3) metal carbonates (magnesium carbonate, calcium carbonate, etc.), (4)metal sulfates (calcium sulfate, barium sulfate, etc.), (5) carbon(carbon black, graphite, diamond, etc.), and (6) clay minerals (kaolin,clay, bentonite, etc.). Out of these, preferred are crosslinked siliconeresin particles, crosslinked polystyrene resin particles,melamine-formaldehyde resin particles, polyamide-imide resin particles,aluminum oxide (alumina) particles, titanium dioxide particles, silicondioxide particles, zirconium oxide particles, synthesized calciumcarbonate particles, barium sulfate particles, diamond particles andkaolin particles. More preferred are crosslinked silicone resinparticles, crosslinked polystyrene resin particles, aluminum oxide(alumina) particles, titanium dioxide particles, silicon dioxideparticles and calcium carbonate particles. The above inert particles maybe used alone or in combination of two or more.

The inert particles to be contained in the film layer on thenon-magnetic layer side have an average particle diameter of preferably0.05 to 1.0 μm, more preferably 0.1 to 0.7 μm, particularly preferably0.1 to 0.6 μm. The amount of the inert particles is preferably 0.01 to 2wt %, more preferably 0.1 to 1 wt %, particularly preferably 0.1 to 0.5wt %. When the average particle diameter is smaller than 0.05 μm or theamount is smaller than 0.01 wt %, slipperiness becomes unsatisfactoryand winding properties and handling properties in the processing stepbecome worse. When the average particle diameter is larger than 1.0 μmor the amount is larger than 2 wt %, the magnetic side becomes roughbecause the flat layer is thrust up by a lubricant contained in therough layer by calendering or the like, or the surface properties of therunning side are transferred to the magnetic side at the time of curing,thereby causing an error. The above inert particles may be used alone orin combination of two or more. As for the type of the inert particles,the same type of inert particles as those added to the magnetic layerside are preferred.

The thickness of the above biaxially oriented film is preferably 2 to 10μm, more preferably 3 to 7 μm, particularly preferably 4 to 6 μm. Whenthe thickness is larger than 10 μm, the length of the obtained magnetictape wound round a cassette becomes short, thereby making it difficultto increase the capacity of the tape. When the thickness of the film issmaller than 2 μm, force applied at the time of starting and stoppingthe magnetic tape causes the permanent elongation of the film, therebymaking it difficult to obtain satisfactory durability. As for thethickness ratio of the magnetic layer to the non-magnetic layer in thecase of a laminated film, the thickness of the non-magnetic layer ispreferably ⅔ or less, more preferably ½ or less, particularly preferably⅓ or less of the total thickness of the laminated biaxially orientedpolyester film.

Out of the above biaxially oriented films produced by the process of thepresent invention, the biaxially oriented film of the present inventionis particularly excellent in physical properties as described below.

The biaxially oriented polyester film has (i) a Young's modulus in thelongitudinal direction of 8 to 12 GPa, (ii) a Young's modulus in thetransverse direction of 6.5 to 9 GPa, (iii) a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient in the transversedirection (αh) of +5×10⁻⁶/% RH to +12×10⁻⁶/% RH, and (v) a thermalshrinkage factor at 105° C. in the transverse direction of 0 to +1.5%and comprises ethylene-2,6-naphthalene dicarboxylate in an amount of atleast 95 mol % of the total of all the recurring units.

The total of Young's moduli in longitudinal and transverse directions ispreferably 15 to 20 GPa.

A film having a higher Young's modulus in the longitudinal directionthan a Young's modulus in the transverse direction is preferred.

A film having a refractive index in the thickness direction (Nz) of lessthan 1.490 is preferred.

Further, a film having a center plane average roughness (WRa) of atleast one side of 0.5 to 10 nm is preferred.

When the above biaxially oriented polyester film of the presentinvention is a laminated film, the laminate preferably consists of twoadjacent layers made from a polyester comprisingethylene-2,6-naphthalene dicarboxylate in an amount of at least 95 mol %of the total of all the recurring units and has a center plane averageroughness (WRa) of one side of 0.5 to 10 nm and a WRa of the other sideof 1 to 20 nm.

The biaxially oriented film of the present invention can be changed intoa metal coated magnetic recording medium for high-density recordingwhich has excellent electromagnetic conversion characteristics such asoutput at a short-wavelength range, S/N and C/N, few drop outs and a lowerror rate by applying a coating solution prepared by uniformlydispersing iron or needle-like fine magnetic powders (metal powders)containing iron as the main component into a binder such as polyvinylchloride or vinyl chloride-vinyl acetate copolymer to the surface havinga lower surface roughness (magnetic layer side) to form a magnetic layerhaving a thickness of preferably 1 μm or less, more preferably 0.1 to 1μm, and optionally further forming a back coat layer on the oppositeside by a known method. A non-magnetic layer may also be formed on thesurface on the magnetic layer side of the film as a layer underlying theabove metal powder-containing magnetic layer by applying a coatingsolution prepared by dispersing fine titanium oxide particles or thelike in the same organic binder as that of the magnetic layer.

The thus obtained metal coated magnetic recording medium can be used asa large-capacity computer tape, particularly an LTO, DLT or Super-DLTmagnetic tape of linear recording system, for a magnetic tape which hasexcellent running properties, durability, dimensional stability andelectromagnetic conversion characteristics. In the biaxially orientedfilm of the present invention, a metal thin film as a magnetic layer canbe used in place of the coating film. In this case, a deposited magneticrecording medium for high-density recording which has excellentelectromagnetic conversion characteristics such as output at ashort-wavelength range, S/N and C/N, few drop outs and a low error ratecan be obtained by forming a ferromagnetic metal thin film layer ofiron, cobalt, chromium or an alloy or oxide essentially composed thereofon the side having a lower surface roughness by vacuum vapor deposition,sputtering, ion plating or the like, forming a protective layer ofdiamond-like carbon (DLC) or the like and a fluorine-containingcarboxylic acid-based lubricant layer on the surface of theferromagnetic metal thin film layer sequentially according to purpose orapplication, and forming a back coat layer on the opposite side(non-magnetic layer) by the above method.

EXAMPLES

The following examples are provided to further illustrate the presentinvention. Various physical properties and characteristic properties inthe present invention were measured and defined as follows.

(1) Young's Modulus

The film is cut to a width of 10 mm and a length of 150 mm, thisobtained sample is pulled by an Instron type universal tensile tester ata chuck interval of 100 mm, a pull rate of 10 mm/min and a chart rate of500 mm/min, and the Young's modulus is calculated from the tangent of arising portion of the obtained load-elongation curve.

(2) Surface Roughness (WRa)

Using the non-contact 3-D roughness meter (NT-2000) of WYKO Co., Ltd.,the surface roughness of the film is measured for 10 or more times (n)under such conditions as a measurement area of 246.6 μm×187.5 μm (0.0462mm²) and a measurement magnification of ×25, and the center planeaverage roughness (WRa) is obtained with surface analysis softwareincorporated in the roughness meter.

$\begin{matrix}{{WRa} = {\sum\limits_{k = 1}^{m}{\sum\limits_{j = 1}^{n}{{{Z_{jk} - \overset{\_}{Z}}}/\left( {m \cdot n} \right)}}}} \\{{{provided}\mspace{14mu}\overset{\_}{Z}} = {\sum\limits_{k = 1}^{m}{\sum\limits_{j = 1}^{n}{Z_{jk}/\left( {m \cdot n} \right)}}}}\end{matrix}$wherein Z_(jk) is a height on a 2-D roughness chart at a j-th positionand a k-th position in a measurement direction (246.6 μm) and adirection perpendicular to the measurement direction (187.5 μm) whenthese directions are divided into m and n sections, respectively.(3) Temperature Expansion Coefficient (αt)

The film sample is cut to a length of 15 mm and a width of 5 mm in thetransverse direction of the film, and the obtained sample is set in theTMA3000 of Shinku Riko Co., Ltd. to be pre-treated at 60° C. in anitrogen atmosphere for 30 minutes and cooled to room temperature.Thereafter, the temperature is raised from 25° C. to 70° C. at a rate of2° C./min and the length of the sample is measured at each temperatureto calculate the temperature expansion coefficient (αt) of the film fromthe following equation.αt={(L ₂ −L ₁)/(L ₀ ×ΔT)}×10⁶+0.5 (note)wherein L₁ is the length (mm) of the sample at 40° C., L₂ is the length(mm) of the sample at 60° C., L₀ is the initial length (mm) of thesample, and ΔT is 60−40=20 (° C.).

-   (Note): temperature expansion coefficient of quartz glass (×10⁶)    (4) Humidity Expansion Coefficient (αh)

The film sample is cut to a length of 15 mm and a width of 5 mm in thetransverse direction of the film, and the obtained sample is set in theTMA3000 of Shinku Riko Co., Ltd. and maintained at a humidity of 20% RHand a humidity of 80% RH from a nitrogen atmosphere to measure thelength of the sample and calculate its humidity expansion coefficientfrom the following equation.αh={(L ₂ −L ₁)×10⁻⁶/(L ₁ ×ΔH)}wherein L₁ is the length (mm) of the sample at a humidity of 20% RH, L₂is the length (mm) of the sample at a humidity of 80% RH, and ΔH is 60(=80−20% RH).(5) Thermal Shrinkage Factor

The film sample cut to a length of 300 mm and a width of 10 mm in thetransverse direction is placed in an oven heated at 105° C. under noload, heated for 30 minutes, taken out from the oven and cooled to roomtemperature to read its dimensional change. The thermal shrinkage factorof the film sample is calculated from its length (L₀) before the heattreatment and a dimensional change (ΔL) by the heat treatment based onthe following equation.thermal shrinkage factor=(ΔL/L ₀)×100(%)In the case of elongation, ΔL is a negative value.(6) Refractive Index

The refractive indices in longitudinal and transverse directions of thefilm are measured at 25° C. using an Abbe refractometer (of Atago Co.,Ltd.) and Na-D rays. Both the front and rear sides of the film sampleare measured and the average value of the measurement data is taken asrefractive index.

(7) Dimensional Change in Crosswise Direction Under Load in LongitudinalDirection at the Time of High-Temperature and High-Humidity Treatment

The film slit to a width of 12.65 mm (½ inch) is set as shown in FIG. 1at an ambient temperature of 23° C. and an ambient humidity of 50% RH.

In FIG. 1, the numerals represent the following.

-   1 measurement sample-   2 light emitting portion of an optical sensor (LS-3036 of Keyence    Co., Ltd.)-   3 light receiving portion of an optical sensor (LS-3036 of Keyence    Co., Ltd.)-   4 load-   5 free roll-   6 glass plate-   7 measuring instrument (LS-3100 of Keyence Co., Ltd.)-   8 analog/digital converter-   9 personal computer-   10 laser beam

Gold has been deposited on the surface of the sample slit to a width of12.65 mm by sputtering so that its width can be measured with adetector. In this state, a weight of 29 MPa per the sectional area ofthe film is attached to one side of the film (the other side is fixed)to measure the width (L₁) of the film with the laser outer diametermeasuring instrument of Keyence Co., Ltd. (body: model 3100, sensor:model 3060).

Thereafter, a weight of 29 MPa per the sectional area of the film isattached to one side of the film (the other side is fixed) at atemperature of 49° C. (120° F.) and a humidity of 90% RH, kept in thisstate for 72 hours (3 days) and removed, and the film was kept at anambient temperature of 23° C. and an ambient humidity of 50% for 24hours. Then, a weight of 29 MPa per the sectional area of the film isattached to one side of the film (the other side is fixed) again tomeasure the width (L₂) of the film with the laser outer diametermeasuring instrument of Keyence Co., Ltd. (body: model 3100, sensor:model 3060).

The dimensional change in the crosswise direction (αW) before and afterthe temperature and humidity treatment under load is calculated from theabove measured sizes before and after the temperature and humiditytreatment based on the following equation.αW={(L ₂ −L ₁)/L ₁}×100(%)

The evaluation criteria are as follows.

(8) Track Dislocation (Error Rate)

The error rate is measured under the following conditions using theML4500B QIC system of Media Logic Co., Ltd.

-   Current: 15.42 mA-   Frequency: 0.25 MHz-   Location: 0-   Threshold: 40.0-   Bad/good/max: 1:1:1-   Tracks: 28

The error rate is the average value of the number of measured tracks.

The error rate is measured under condition 1 (track dislocation causedby temperature and humidity variations) and under condition 2 (trackdislocation caused by a temperature and humidity treatment) as follows.condition 1 (track dislocation caused by temperature and humidityvariations):

-   A tape which recorded data at 10° C. and 10% RH is reproduced at a    temperature of 45° C. and a humidity of 80% RH to measure the amount    of track dislocation caused by temperature and humidity variations.    The measurement result is evaluated based on the amount of track    dislocation of the sample of Example 1 according to the following    criteria.-   ⊚: error rate is zero-   ◯: error rate is low and there is no practical problem-   X: error rate is high and there is a practical problem condition 2    (track dislocation caused by temperature and humidity treatment)

A tape which recorded data at 23° C. and 50% RH is caused to run at 40°C. and 60% RH repeatedly for 60 hours and then kept at 23° C. and 50% RHfor 24 hours, and then data is reproduced at 23° C. and 50% RH tomeasure the amount of track dislocation caused by a temperature andhumidity treatment.

The measurement result is evaluated based on the amount of trackdislocation of the sample of Example 1 according to the followingcriteria.

-   ⊚: error rate is zero-   ◯: error rate is low and there is no practical problem-   X: error rate is high and there is a practical problem    (9) Electromagnetic Conversion Characteristics of Magnetic Tape

The electromagnetic conversion characteristics of a magnetic tape aremeasured with the ML4500B QIC system of Media Logic Co., Ltd. Themeasurement result is evaluated based on the following criteria when theS/N of the sample of Example 1 is 0 dB.

-   ⊚: +1 dB or more-   ◯: −1 dB or more and less than +1 dB-   X: less than −1 dB

Example 1

Polyethylene-2,6-naphthalate (intrinsic viscosity: 0.6) containing 0.02wt % of calcium carbonate particles having an average particle diameterof 0.6 μm and 0.2 wt % of silica particles having an average particlediameter of 0.1 μm was dried at 180° C. for 5 hours, melt extruded at300° C. and solidified by quenching on a casting drum maintained at 60°C. to obtain an unstretched film.

This unstretched film was stretched to 6.2 times in a longitudinaldirection between two rolls having different speeds at 150° C. Theuniaxially oriented film after longitudinal stretching had a refractiveindex in the longitudinal direction of more than 1.77, an refractiveindex in the transverse direction of 1.587 and a refractive index in thethickness direction of 1.534. This uniaxially oriented film wasstretched to 4.5 times in the transverse direction at a draw rate of87.5%/sec while the temperature was raised to 120 to 155° C. in thetraveling direction of the film, further stretched to 1.1 times in thetransverse direction again at a draw rate of 2.9%/sec while thetemperature was raised to 155 to 205° C. in the traveling direction ofthe film and heat set at 190° C. in the final heat setting zone for 5seconds while it was toed out by 5% (1.05 times). The obtained biaxiallyoriented film had a thickness of 4.5 μm.

The following composition was placed in a ball mill and kneaded for 16hours to be dispersed, and 5 parts by weight of an isocyanate compound(Desmodule L of Bayer AG) was added and dispersed by high-speed shearingfor 1 hour to prepare a magnetic coating.

composition of magnetic coating: needle-like Fe particle 100 parts byweight vinyl chloride-vinyl acetate copolymer  15 parts by weight (Eslec7A of Sekisui Chemical Co., Ltd.) thermoplastic polyurethane resin  5parts by weight chromium oxide  5 parts by weight carbon black  5 partsby weight lecithin  2 parts by weight fatty acid ester  1 part by weighttoluene  50 parts by weight methyl ethyl ketone  50 parts by weightcyclohexanone  50 parts by weight

This magnetic coating was applied to one side of the above biaxiallyoriented PEN film to ensure that the final thickness of the coatinglayer should become 0.5 μm, and the obtained film was oriented in 2,500Gauss of a DC magnetic field, dried by heating at 100° C.,supercalendered (linear pressure of 200 kg/cm, temperature of 80° C.)and rolled. This roll was left in an oven heated at 55° C. for 3 days.

The following back coat was applied to the other side of the biaxiallyoriented PEN film to ensure that the thickness of the coat should become1 μm, and the obtained film was dried and cut to obtain a magnetic tape.

composition of back coat carbon black  100 parts by weight thermoplasticpolyurethane resin   60 parts by weight isocyanate compound   18 partsby weight (Colonate L of Nippon Polyurethane Kogyo Co., Ltd.) siliconeoil  0.5 part by weight methyl ethyl ketone  250 parts by weight toluene  50 parts by weight

The characteristic properties of the thus obtained film and magnetictape are shown in Table 1. As obvious from Table 1, the obtained tapehad excellent dimensional stability in the crosswise direction(temperature and humidity variations and high-temperature andhigh-humidity treatment under load in longitudinal direction) andexcellent output characteristics and was free from track dislocation.

Example 2

The film was stretched to 6.2 times in the longitudinal direction at150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.534. Thisuniaxially oriented film was stretched to 4.2 times in the transversedirection at a draw rate of 80.0%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, furtherstretched to 1.20 times in the transverse direction again at a draw rateof 5.8%/sec while the temperature was raised to 155 to 205° C. in thetraveling direction of the film, and heat set at 190° C. in the finalheat setting zone for 5 seconds while it was toed in (0.95 time) by 5%.The obtained biaxially oriented film had a thickness of 4.5 μm.

A magnetic tape was obtained from the thus obtained film in the samemanner as in Example 1. The characteristic properties of the film andmagnetic tape are shown in Table 1. As obvious from Table 1, theobtained tape had excellent dimensional stability in the crosswisedirection (temperature and humidity variations and high-temperature andhigh-humidity treatment under load in longitudinal direction) andexcellent output characteristics and was free from track dislocation.

Example 3

Polyethylene-2,6-naphthalate (intrinsic viscosity: 0.6) for layer Bwhich contained 0.15 wt % of crosslinked silicone resin particles havingan average particle diameter of 0.3 μm and 0.15 wt % of spherical silicaparticles having an average particle diameter of 0.1 μm andpolyethylene-2,6-naphthalate (intrinsic viscosity: 0.6) for layer Awhich contained 0.01 wt % of spherical silica particles having anaverage particle diameter of 0.1 μm were prepared, and pellets of thesepolyethylene-2,6-naphthalates were dried at 180° C. for 5 hours,supplied to the respective hoppers of two extruders, molten at atemperature of 300° C., laminated together using a multi-manifoldcoextrusion die in such a manner that the layer A was placed on one sideof the layer B, and extruded onto a casting drum having a surface finishof about 0.3 S and a surface temperature of 60° C. to obtain anunstretched laminated film. The thickness of each layer was adjusted bythe delivery rates of the two extruders to achieve surface roughnessshown in Table 1.

This unstretched film was stretched to 6.0 times in the longitudinaldirection between two rolls having different speeds at 150° C. Afterlongitudinal stretching, the uniaxially oriented film had a refractiveindex in the longitudinal direction of more than 1.77, a refractiveindex in the transverse direction of 1.587 and a refractive index in thethickness direction of 1.536. This uniaxially oriented film wasstretched to 4.8 times in the transverse direction at a draw rate of95.0%/sec while the temperature was raised to 120 to 155° C. in thetraveling direction of the film, further stretched to 1.15 times in thetransverse direction again at a draw rate of 4.4%/sec while thetemperature was raised to 155 to 205° C. in the traveling direction ofthe film and heat set at 190° C. in the final heat setting zone for 5seconds by making the rails straight (1.00 time). The obtained biaxiallyoriented film had a thickness of 4.5 μm.

A magnetic coating was applied to the surface of the layer A (magneticlayer side) and a back coat was applied to the surface of the layer B(non-magnetic layer side) in the same manner as in Example 1, dried andcut to obtain a magnetic tape.

The characteristic properties of the thus obtained film and magnetictape are shown in Table 1. As obvious from Table 1, the obtained tapehad excellent dimensional stability in the crosswise direction(temperature and humidity variations and high-temperature andhigh-humidity treatment under load in longitudinal direction) andexcellent output characteristics and was free from track dislocation.

Comparative Example 1

The film was stretched to 6.2 times in the longitudinal direction at150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.534. Thisuniaxially oriented film was stretched to 2.6 times in the transversedirection at a draw rate of 40.0%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, furtherstretched to 2.00 times in the transverse direction again at a draw rateof 29.2%/sec while the temperature was raised to 155 to 205° C. in thetraveling direction of the film and heat set at 190° C. in the finalheat setting zone for 5 seconds by making the rails straight (1.00time). Since the film was often broken by the second transversestretching, a roll sample could not be obtained.

Comparative Example 2

The film was stretched to 6.0 times in the longitudinal direction at150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.536. Thisuniaxially oriented film was stretched to 5.5 times in the transversedirection at a draw rate of 112.5%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, the rails weremade straight (1.00 time) while the temperature was raised to 155 to205° C. in the traveling direction of the film, and the film was furtherheat set at 190° C. in the final heat setting zone for 5 seconds bymaking the rails straight (1.00 time). Since the film was often brokenby the first transverse stretching, a roll sample could not be obtained.

Comparative Example 3

The film was stretched to 6.2 times in the longitudinal direction at150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.534. Thisuniaxially oriented film was stretched to 4.3 times in the transversedirection at a draw rate of 82.5%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, furtherstretched to 1.10 times in the transverse direction again at a draw rateof 2.9%/sec while the temperature was raised to 155 to 205° C. in thetraveling direction of the film and heat set at 190° C. in the finalheat setting zone for 5 seconds while it was toed out by 10% (1.10times). The obtained biaxially oriented film had a thickness of 4.5 μm.

A magnetic tape was obtained from the thus obtained film in the samemanner as in Example 1. The characteristic properties of the film andmagnetic tape are shown in Table 1. As obvious from Table 1, theobtained film had a large thermal shrinkage factor in the transversedirection and poor dimensional stability in the crosswise direction ofthe tape (high-temperature and high-humidity treatment under load inlongitudinal direction).

Comparative Example 4

The film was stretched to 5.7 times in the longitudinal direction at150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.539. Thisuniaxially oriented film was stretched to 3.9 times in the transversedirection at a draw rate of 72.5%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, the rails weremade straight (1.00 time) while the temperature was raised to 155 to205° C. in the traveling direction of the film, and the film was furtherheat set at 190° C. in the final heat setting zone for 5 seconds bymaking the rails straight (1.00 time). The obtained biaxially orientedfilm had a thickness of 4.5 μm.

A magnetic tape was obtained from the thus obtained film in the samemanner as in Example 1. The characteristic properties of the film andmagnetic tape are shown in Table 1. As obvious from Table 1, theobtained film had a low Young's modulus in the transverse direction andpoor dimensional stability in the crosswise direction of the tape(temperature and humidity variations).

Comparative Example 5

The film was stretched to 4.0 times in the longitudinal direction at150° C. in Example 1. After longitudinal stretching, the uniaxiallyoriented film had a refractive index in the longitudinal direction ofmore than 1.77, a refractive index in the transverse direction of 1.587and a refractive index in the thickness direction of 1.558. Thisuniaxially oriented film was stretched to 5.4 times in the transversedirection at a draw rate of 110.0%/sec while the temperature was raisedto 120 to 155° C. in the traveling direction of the film, the rails weremade straight (1.00 time) while the temperature was raised to 155 to205° C. in the traveling direction of the film, and the film was furtherheat set at 190° C. in the final heat setting zone for 5 seconds bymaking the rails straight (1.00 time). The obtained biaxially orientedfilm had a thickness of 4.5 μm.

A magnetic tape was obtained from the thus obtained film in the samemanner as in Example 1. The characteristic properties of the film andmagnetic tape are shown in Table 1. As obvious from Table 1, theobtained film had a low Young's modulus in the transverse direction andpoor dimensional stability in the crosswise direction of the tape(high-temperature and high-humidity treatment under load in longitudinaldirection).

TABLE 1 Item unit Ex. 1 Ex. 2 Ex. 3 C. Ex. 1 C. Ex. 2 C. Ex. 3 C. Ex. 4C. Ex. 5 layer structure single single double single single singlesingle single layer layer layers layer layer layer layer layer filmforming conditions draw ratio in longitudinal times 6.2 6.2 6.0 6.2 6.06.2 5.7 4.0 direction draw ratio in transverse direction first drawratio (SD1) times 4.5 4.2 4.8 2.6 5.5 4.3 3.9 5.4 second draw ratio(SD2) times 1.10 1.20 1.15 2.00 1.00 1.10 1.00 1.00 ratio in final heattimes 1.05 0.95 1.00 1.00 1.00 1.10 1.00 1.00 setting zone (SD3) totalratio (SD1 × SD2 × SD3) times 5.20 4.79 5.52 5.20 5.50 5.20 3.9 5.40draw rate first draw rate (S1) %/sec 87.5 80.0 95.0 40.0 112.5 82.5 72.5110.0 second draw rate (S2) %/sec 2.9 5.8 4.4 29.2 0.0 2.9 0.0 0.0 S1/S2— 0.03 0.07 0.05 0.73 0.00 0.04 0.00 0.00 film formation state firsttransverse film is stretching zone often broken second transverse filmis stretching zone often broken Physical properties Film thickness μm4.5 4.5 4.5 4.5 4.5 4.5 4.5 4.5 Young's modulus Longitudinal directionGPa 9.0 9.7 8.5 9.0 8.5 9.0 9.5 6.0 Transverse direction GPa 7.0 6.5 7.57.0 7.5 7.0 5.5 9.0 Refractive index in thickness 1.484 1.487 1.4841.484 1.484 1.484 1.487 1.487 direction (biaxially oriented film)Temperature expansion coefficient (αt) Transverse direction ×10⁻⁶/° C. 38 0 3 0 3 17 −5 Humidity expansion coefficient (αh) Transverse direction×10⁻⁶/% RH 12 13 10 12 10 12 16 8 105° C. thermal shrinkage factortransverse direction % 1.3 0.3 0.8 2.3 0.5 1.8 0.3 0.3 dimensionalchange in crosswise direction % 0.20 0.17 0.22 0.20 0.22 0.20 0.18 0.35under load in longitudinal direction caused by high-temperature andhigh-humidity treatment surface roughness magnetic layer side (WRa) nm 66 2 6 6 6 6 6 non-magnetic layer side nm 6 6 8 6 6 6 6 6 trackdislocation condition 1 (track dislocation caused ⊚ ◯ ⊚ — — ⊚ X ⊚ bytemperature and humidity variations) condition 2 (track dislocationcaused ◯ ⊚ ◯ — — X ⊚ X by temperature and humidity variations)electromagnetic conversion characteristics ◯ ◯ ⊚ — — ◯ ◯ ◯ Ex.: ExampleC. Ex.: Comparative Example

1. A biaxially oriented polyester film having (i) a Young's modulus in alongitudinal direction of 8 to 12 GPa, (ii) a Young's modulus in atransverse direction of 6.5 to 9 GPa, (iii) a temperature expansioncoefficient in the transverse direction (αt) of −5×10⁻⁶/° C. to+12×10⁻⁶/° C., (iv) a humidity expansion coefficient in the transversedirection (αh) of +6×10⁻⁶/% RH to +12×10⁻⁶/% RH and (v) a thermalshrinkage factor at 105° C. in the transverse direction of 0 to +1.5%and comprising (vi) ethylene-2,6-naphthalene dicarboxylate in an amountof at least 95 mol % of the total of all the recurring units.
 2. Thefilm of claim 1 which has a total of Young's moduli in the longitudinaland transverse directions of 15 to 20 GPa.
 3. The film of claim 1 whichhas a higher Young's modulus in the longitudinal direction than aYoung's modulus in the transverse direction.
 4. The film of claim 1which has a refractive index in a thickness direction (Nz) of less than1.490.
 5. The film of claim 1 which has a center plane average roughness(WRa) of at least one side of 0.5 to 10 nm.
 6. The film of claim 1 whichis a laminate comprising two layers of a polyester comprisingethylene-2,6-naphthalene dicarboxylate in an amount of at least 95 mol %of the total of all the recurring units and has a center plane averageroughness (WRa) of one side of 0.5 to 10 nm and a WRa of the other sideof 1 to 20 nm.
 7. A magnetic recording medium comprising the biaxiallyoriented polyester film of claim 1 as a base film.
 8. The magneticrecording medium of claim 7 which is a digital recording medium of alinear recording system.
 9. The magnetic recording medium of claim 7which is a coated magnetic recording medium.
 10. The magnetic recordingmedium of claim 7 which is a ferromagnetic metal thin film magneticrecording medium.
 11. A magnetic recording medium comprising thebiaxially oriented polyester film of claim 1 and a magnetic layer formedon the surface of the film.